In recent years, a small portable terminal typified by a smartphone and a tablet, etc. is rapidly spread, which increases the demand for a battery having a small size and a high energy density for driving this terminal.
In general, a graphite material is used for a negative electrode of a lithium ion secondary battery. The theoretical capacity of a graphite material is 372 mAh/g (LiC6), and at the present, the achieved capacity is approaching this limit. In order to further improve an energy density of a lithium ion secondary battery, it becomes necessary to select a new material. From this viewpoint, silicon and tin, which have the large specific capacity and the next lower potential than carbon and lithium, have attracted attention.
Of these materials, silicon can absorb 4.4 lithium atoms per 1 silicon atom in molar ratio, and theoretically, it is possible to obtain about 10 times the capacity of a graphite-based carbon material. However, when a silicon particle absorbs lithium, the volume thereof expands to about 3 to 4 times the original volume, and thus, there is the problem that repeated charge and discharge causes the deterioration, which results in the capacity reduction. This phenomenon was carefully analyzed, and the following was confirmed. When lithium is inserted into a silicon-containing active material, the volume expansion causes fine cracks in an electrode, and an electrolyte solution enters through these fine cracks, which forms a new coating film (SEI layer). In this process, an irreversible capacity is generated, which consequently reduces a battery capacity. This phenomenon appears as the change of a charge and discharge efficiency during cycles. The reduction of a cycle efficiency at an initial stage of cycles, at which volume change is particularly large, has a significant effect on the life of a battery including a positive electrode having a high charge and discharge efficiency. Therefore, when using a silicon-containing active material, it is the important issue to minimize the change of an electrode structure caused by the aforementioned volume expansion.
Examples of the method of dispersing and relieving the stress caused by volume expansion include the method of providing a large number of voids in an active material layer constituting an electrode. In the method of preparing a porous active material layer and connecting voids in a three-dimensional manner, it is possible to obtain a certain effect on the relief of volume expansion, but there are the problems of the decrease in electrode strength and the inability to increase a volume energy density. In addition, there is the proposed method of preliminarily patterning an active material layer so as to form grooves and to divide an electrode into small regions. However, in this method, the groove part does not include an active material, and thus, there is the problem of the decrease in the energy density per unit volume of an electrode.